Institute of Mineralogy Research Research projects
Transport and reactions of light elements (Li, B) in pegmatitic systems under thermal disequilibrium - implications for magmatic/hydrothermal ore deposits

Transport and reactions of light elements (Li, B) in pegmatitic systems under thermal disequilibrium - implications for magmatic/hydrothermal ore deposits

Led by:  Prof. Dr. Harald Behrens, Prof. Dr. Stefan Weyer
Team:  M. Sc. Christian Ronny Singer
Year:  2020
Funding:  DFG
Duration:  2020-2023

Lithium and boron are important geochemical tracers for hydrothermal and magmatic differentiation processes. In addition to the distribution of these elements within rocks, the isotopic abundance may be a useful tool to get insights into fluid/rock interaction, possible sources of fluids and the evolution of magmas. Due to the relatively large mass difference of the most common isotopes of Li and B (7Li/6Li; 11B/10B) large fractionation effects may occur, driven by equilibrium partitioning between coexisting phases or by kinetic (non-equilibrium) processes. While the knowledge of partitioning of Li and B isotopes between coexisting phases has made significant progress in the last decades, the knowledge of the behavior of these elements under disequilibrium conditions is still very poor. In the present project we want to investigate transport and reactions of light elements, Li and B, in pegmatitic systems under controlled disequilibrium conditions. The major focus will be on lithium and its isotopes, since this element is very mobile, i.e. in presence of fluids. Another motivation is the importance of Li-rich pegmatitic ore deposits as possible lithium sources. Boron will be studied for comparison. The diffusivity of B in melts is typically orders of magnitude lower than that of Li, but in fluids B may be transported fast as well. The main goal of the proposal is the experimental investigation of the interaction of fluids with melts and minerals in a temperature gradient. About 10 cm long gold capsules will be loaded in a vertical cold seal pressure vessel and inserted in a three-zone furnace. The planned assembly consists of a hydrous pegmatitic melt (containing Li and B) on top, quartz powder with embedded pre-fractured quartz crystals (to trap the fluid in inclusions) in the center and a sink for Li and B (muscovite or Li,B free pegmatite melt) on bottom. At pressures of 1-2 kbar, temperature differences of 0 - 100 K over the sample length will be adjusted with maximum temperature to be in the range of 600 - 800°C. Fluids will consist either of pure H2O, or H2O containing 5 - 10 wt% NaCl. UV-femtosecond laser ablation coupled with mass spectrometry will be used for in situ measurement of element concentrations and isotope ratios in the glass (quenched melt) and muscovite as well as fluid inclusions in quartz. These analyses will allow constraining the partitioning behavior of Li and B and their isotopes between melt and fluid, as well as isotope fractionation during diffusion in melt and fluid. Variation of parameters (duration, fluid composition, pressure, maximum temperature, T-gradient) will be used to evaluate the importance of the different steps in the overall process. Application of the findings to selected pegmatites and contact aureoles will be tested in cooperation with other researchers. Data are compared with results of other studies of the SPP e.g. on equilibrium partitioning of elements and isotopes in pegmatite-like systems.